As Real As It Gets: TH350

It’s easy to get caught up in the headlines of big horsepower and all of the glamour and glory that goes along with it. Supercharged and turbocharged engines making over 1,000 hp on pump gas are the hottest rage right now. Monster big-blocks and super fancy LS engines seem to be in every driveway across the land.

Naturally, a top-of-the-line TH400 or high-end exotic Overdrive transmission back one of these unbelievable powerplants, or maybe even one of those fancy six-speed manual transmissions has found its way into the trans tunnel. The modern era of high-performance has brought us to the brink of insanity. That insanity, unfortunately, is not available in every driveway no matter how the wealth is redistributed in this country. For most of us those combinations aren’t realistic and won’t be finding their way into any of our street cars anytime soon.

Odds are most readers employ a really nice big- or small-block built on a workingman’s budget. These real-world engine combinations don’t require a transmission with a royal pedigree. We are also willing to bet that a three-speed TH350 is more than capable of harnessing most popular packages. If you have to leave the exotic high-end stuff for the other side of the Super Chevy marketplace, give Performance Automatic a ring. The company is making it easy to swap the popular TH350 transmission into your Chevy with its Street Smart package.

The Street Smart TH350 Stage 1 (PN PA35101)—transmission only—shows a $1,387 retail price tag, but no one pays retail anymore. We’ve seen several mail-order companies advertise Stage 1 for around $1,100—a number that won’t break the bank. The Stage 1 TH350 is rated up to 450 hp. Those seeking something a little stronger can check out the Stage 2 version (PN PA35102) that holds 550 hp and retails for $1,500 with a real world street price of $1,175 from most vendors. Both transmissions come with a standard one-year warranty.

Performance Automatic also offers a complete package that includes a torque converter matched to your combination, transmission dipstick, and a universal transmission mount. The Stage 1 complete package (PA53103) is listed for $1,750 but the three vendors we looked up online had the Street Smart TH350 Stage 1 package listed for around $1,350. The Stage 2 Package (PA35104) bumped the price tag up to $1,790 from those same dealers. A lifetime warranty is included in the Stage 2 Package.

Backing up for a moment, the TH350 came into existence in the late ’60s as a replacement for the popular Powerglide. The biggest difference between the two, besides internal workings, is the TH350’s three forward speeds verse just two speeds of the Powerglide. The three-speed automatic transmission enjoyed a service life of nearly 20 years as it was stuffed behind small-blocks and V-6 engines in most rear-wheel drive cars and trucks through 1984. It is safe to say production numbers are in the millions. That makes the cores common and with some minor upgrades these transmissions have proven to be versatile in a wide-variety of applications.

“Our TH350 Street Smart package comes as Stage 1 or Stage 2 versions. We have customers running them in their street cars, street rods, pro-touring/autocross vehicles, and it sees a lot of street/strip uses,” comments Harvey Baker of Performance Automatic. He invited us down to Maryland to take an in-depth tour of one the company’s most popular products. We jumped at the chance to follow Performance Automatic’s Gerald Nusser as he cracked open a TH350 and rebuilt it into a high-performance automatic transmission.

The build up is what you’d expect as each part is cleaned and inspected with the major items being replaced with stronger and better parts. Nusser was barely into the teardown when we came across one of the most common problems—a broken accumulator spring. In some cases the cracked spring will hurt the bore or accumulator so each is inspected and repairs are made, if necessary. The front pump is pulled apart and inspected. Nusser says to pay particular attention to the wearable pieces of the front pump and replace anything that shows signs of wear marks. Every pump is outfitted with new seals and bearings regardless of its condition.

As Nusser tore deeper and deeper into the guts, he showed us more upgrades. All clutches are replaced with high-performance Raybestos brand clutches and the stock band is tossed in favor of a Kevlar one.

As for clutch count, Nusser tells us that the factory supplies either three or four clutches, depending on the year of the transmission. Performance Automatic installs five clutches for better holding strength in Stage 2 units. The direct piston has to be modified in order to fit the extra clutches. The factory piston is put on a lathe and Nusser removes some material for the extra clearance.

We asked what are some common problems with the TH350 and he shared some interesting facts. Surprisingly, the transmission doesn’t have too many issues, but there are some inherent design flaws when this transmission is used in high-performance applications. First on the list is the race that rests between the high-gear drum and the secondary drum.

“The intermediate/Second gear clutch pack race is an issue. The race will shatter or break. We add a hardened one to eliminate this problem,” comments Nusser. “The clutches hold up pretty well in these transmissions. You don’t see a lot of wear at high power levels. Of course, a good shift-kit and valve body helps too.”

Another area he cautioned can be a problem in a high horsepower application is the OEM sprags. Like the race, it is susceptible to cracking and shattering.

Performance Automatic recommends going to a TH400 or Powerglide for applications over 550 hp. But for most enthusiasts, the 550 hp rating of Stage 2 is more than adequate.

Follow along as Nusser shows us the guts and glory of a Performance Automatic TH350 Stage 2 transmission.

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As Real As It Gets: TH350

We shadowed Performance Automatic’s Gerald Nusser as he rebuilt a TH350 into a Stage 2 transmission.

1|There are those out there who have the skills to perform their own transmission rebuilds and if you are capable of doing so, Performance Automatic offers DIY Overhaul kits. For $185 you can walk away with a Max Performance overhaul kit, which is the Performance Automatic Stage 1 transmission. Throwing an additional $105 on top of the Stage 1 price tag will get you the Pro Max Performance overhaul kit for $290. The Pro Max Performance is a Stage 2 overhaul kit.

2|Nusser removes the governor gear and retainer as he prepares to drain the fluid and disassemble the TH350 transmission

3|The stainless steel bench top gets covered with transmission fluid as the TH350 is turned on its side to help drain all of the fluid. The wrenches start turning once the fluid is drained and the transmission comes apart quickly.

4|The valvebody is exposed with the removal of the pan.

5|One of the most common problems in a TH350 is a broken accumulator spring. This one is fine but if you find a broken spring in your transmission be sure to inspect the bore. Performance Automatic discards the stock spring and adds an upgraded one. The accumulator spring is designed to cushion the shift. Nusser points out that Performance Automatic doesn’t block off the accumulator spring in street transmissions. Instead it relies on the valvebody modifications to provide firmer shifts.

6|Performance Automatic adds an extra clutch to the direct drum for Stage 2 builds, which requires the direct piston to be machined down in order to clear the extra thick clutch package. Nusser puts the piston on the lathe and removes 0.170-inches.

7|Here is a comparison of the modified direct piston (front) and the factory one (rear).

8|The case is emptied and cleaned out. Once Nusser cleans the case, it is inspected and verified to be in good condition.

9|The internal parts are also inspected and cleaned in a hot tank industrial washer.

10|Performance Automatic installs five clutches in the case (pictured is the forward drum) and for those DIY mechanics, never put in dry clutches. Rather, dip them thoroughly in transmission fluid before installation. Performance Automatic uses Raybestos clutches in all of its transmission rebuilds.

11|The high-gear drum gets a hardened race on the backside to help durability. The notches are where the intermediate/Second gear clutch pack locks into the high-gear drum. This is one of the problem areas that Performance Automatic fixes for better durability in high-performance applications.

12|The planetary gear-sets are inspected before and after cleaning to ensure proper working order. One thing Nusser looks for is scorching on the gears or heat problems, chipped teeth, different shim problems, and any rocking of the gear (needle-bearing problem).

13|The forward and high gear drum get lowered into the case.

14|On the left is the high-performance Kevlar band while the right is a standard band. All Performance Automatic TH350 transmissions come with the Kevlar band upgrade.

15|The front pump is disassembled, inspected for wear, and rebuilt using new seals and bushings. If there is any sign of wear it is discarded and a new one replaces it.

16|The front pump bolts back into the front of the transmission easily.

17|Here is the accumulator and it is installed prior to the valvebody being bolted back on.

18|The valvebody is nothing more than a fluid control system and a set of check balls is used to help keep control. Some people tend to put the check balls in different areas to help change the shift firmness. Nusser puts the check balls back into the factory locations and Performance Automatic modifies the valvebody for firmer shifts.

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19|The tailshaft housing bolts back on.

20|The valvebody separator plate is drilled for firmer shifting.

21|A Performance Automatic oil pan is added to the mix for enhanced appearance and more fluid capacity.

22|The Street Smart Kit comes with a universal transmission mount.

23|It also comes with a torque converter that is matched to your combination.

24|The transmission is painted before being shipped out the door. There are a variety of color options available as long as it’s black.